Parts coated with thick coating compositions of uni- and polymodal types

ABSTRACT

A coated metal or ceramic part having improved salt corrosion and oxidation resistance having a liquid binder which comprises phosphate ions and ions of the group of chromate or molybdate ions, and an atomized aluminum powder having an average particle size (as expressed in terms of the median equivalent spherical diameter (ESD)), that is greater than at least 15 μm, and having a size distribution such that at least about 5% of the particles by weight are retained on a 325 mesh screen.

This is a division, of application Ser. No. 566,514, filed Dec. 29,1983, now U.S. Pat. No. 4,617,056.

This invention relates to a thick film aluminum-ceramic compositematerial which can replace thermal sprayed aluminum as an abradable ormachinable, corrosion and oxidation resistant coating for metal andceramic parts. The invention also relates to compositions suitable formaking such coatings and to coated parts.

The coating compositions of this invention, in one embodiment, comprisea liquid binder, particularly an aqueous binder which comprises chromateand phosphate ions and a type of atomized aluminum powder with anaverage particle size greater than about 15 μm and having somepopulation fraction greater than 44 microns in diameter (+325 mesh).

The invention also provides in another embodiment a coating of enhancedphysical properties. This composition comprises aluminum powder with anaverage particle size of 15 μm, some fraction of which is +325 mesh, towhich is added a quantity of a second grade of smaller aluminum powderspossessing an average particle diameter about 0.5 or less that of thediameter of the larger grade. The cured film or coating of thiscomposition has improved density and bond strength. The aluminum-filledslurry so described will, when thermally cured, produce an aluminumcoating possessing adhesion, corrosion resistance and oxidationresistance that are superior to those of a thermal sprayed aluminumcoating.

Whenever in this description of the invention the term "aluminum" isused in describing the powder, it is used to describe a preferred metal,and throughout the description there is intended to include metalpowders in general, preferably aluminum.

For convenience in the description of the invention the compositions(and coatings) which comprise one type (or grade) of metal powder of thelarger size, as described above, as designated as unimodal; thecompositions (and coatings) which comprise also the second type (orgrade) of smaller metal powder, designated as poly- or bimodal.

The term "thermal spray" has been used to describe any process whereby amaterial is brought to its melting point and sprayed onto a surface toproduce a coating. These processes now include: (1) Metallizing or WireSpray; (2) Arc Wire Spray; (3) Thermo Spray; and (4) Plasma Flame Spray.

The terms "metallizing" or "wire spray" are used to describe a type ofthermal spraying process which involves the use of metal in wire form.The wire is drawn through the gun and nozzle by a pair of powered feedrollers. Here the wire is continually melted in an oxygen-fuel-gas flameand atomized by a compressed air blast which carries the metal particlesto the previously prepared surface. The individual particles mesh toproduce a coating of the desired metal. This meshing action is still notcompletely understood, but the effect is apparently due to a combinationof mechanical interlocking and cementation of the oxides formed duringthe passage of the particles from the gun nozzle to the sprayed surface.

In the "arc wire spray" process, metal is melted by an electric arc,which is produced by passing current through two converging metal wires,before being atomized in compressed air. Metallizing and arc wire sprayare suitable for producing coatings from any material that can be drawninto wire form.

Materials that cannot be produced in wire form are sprayed using "thermospray" and "plasma flame spray" processes which utilize metals and othermaterials in powder form. In the "thermo spray" process, these powderedmaterials are held in a hopper atop the gun and gravity fed into the gunproper where they are picked up by the oxy-acetylene (or hydrogen) gasmixtures and carried to the gun nozzle. Here they are melted almostinstantly due to the extremely high thermal efficiency of the gun andcarried to the surface being sprayed by means of a siphon-jetarrangement at the gun nozzle. This also contributes to the thermalefficiency of the thermo spray process. Very high deposit efficienciescan be attained (usually well above 90%).

In plasma flame spraying, powders are melted in a plasma instead of anoxy-acetylene flame. The spray gun utilizes an electric arc containedwithin a water cooled jacket. An inert gas, passed through the arc, isexcited to temperatures approaching 30,000° F. This plasma instantlymelts any powder, even refractory materials, as it passes through thegun. These processes are more completely described in a pamphletentitled "The METCO Flame Spray Process", available from METCO Inc.,Westbury, N.Y., which is incorporated herein by reference.

Thermal-sprayed aluminum coatings can be applied by any of the fourmethods described above and are used to restore dimensional toleranceson a metal part or to improve its corrosion and oxidation resistance.For example, the internal surfaces of a cast aluminum diesel engineturbocharger inlet housing are repaired and recontoured using aluminumor aluminum alloy coatings. Approximately 0.045 inches (1.14 mm) ofaluminum is thermo sprayed inside the housing, then the part isremachined to original tolerances. This repair is described in detail inthe METCO Application Bulletin No. 403, which is incorporated herein byreference.

In another application, 0.003 to 0.010 inches (0.08 to 0.25 mm) of wireor arc wire sprayed aluminum is used by the U.S. Navy to preventshipboard corrosion of steel structures. The pure aluminum coatingprovides sacrificial protection to the steel and can be painted to matchother hardware. This protective system is described in the CorrosionControl Manual for DD-963 Class (NAVSEA S9630-AB-MAN-010), which isincorporated herein by reference.

The oxidation resistance and machinability of thermal sprayed aluminumcoatings also make them uniquely suitable as air seals in aircraftturbine engines. In the axial compressor section of the turbine, intakeair is accelerated and compressed by the action of thousands of spinningblades. It is essential for efficiency that the air passing through thecompressor not be able to slip by the blades through spaces between theblade tips and the inside wall of the compressor case, or slip throughspaces between the individual stages (rows) of blades. Air loss at bladetips has been eliminated in General Electric's CF-6 engine series byapplying about 0.020 inches (0.51 mm) of aluminum on the inside of thecase. The spinning blades gouge or abrade a groove in this plasma flamesprayed coating producing a tight seal between the blade tip and casewall. Other materials, such as fibrous metal felts or honeycombs, whichare strong and adherent while offering little resistance to the cuttingaction of the blades, could be used. However, this aluminum seal isrelatively inexpensive and is stable at temperatures as high as the 900°F. achieved in some later compressor stages. This application of plasmaflame sprayed coatings is detailed in GE Specifications B50TF56 and B50TF57, which are incorporated herein by reference.

There are, however, some inherent limitations in thermal sprayedaluminum coatings which can adversely affect their performance in anyapplication. Firstly, coatings which are less than about 0.005 inches(0.13 mm) thick are porous enough to allow moisture to reach thecoating-substrate interface. This is highly undesirable and adverselyaffects the properties of the coatings. Research conducted by the U.S.Navy demonstrated that, on carbon steel, wire sprayed aluminum coatingsthat were 0.004 inches (0.10 mm) thick exhibited poor corrosionresistance in salt spray tests because the porosity of the thin coatingsaccelerated the sacrificial reaction of the aluminum. These findingswere published in the technical manual, External Preservation of SteamValves using Wire Spray Aluminum (NAVSEA S6435-AE-MAN- 010/W SPRAYED CTTor NAVAIR 50-20-1), which is included herein by reference. As aconsequence of this study, the Navy requires that all thermal sprayedaluminum, corrosion resistant coatings be applied at least 0.005 inches(0.13 mm) thick (cf. Corrosion Control Manual referenced above).

Secondly, the bond at the thermal sprayed coating/substrate interface isso sensitive to the condition or characteristics of that interface thatit is often difficult to assure good adhesion of the coating to a part.In order to produce a well-bonded coating, the metal or ceramic surfacemust be clean and rough. A clean surface is one free of dirt, oil,moisture and all other contamination. Even the transparent oxide filmthat forms on a steel part at room temperature is enough to preventadhesion of the thermal sprayed coating. Therefore, the U.S. NavyCorrosion Control Manual cited above requires that any surface notcoated within two hours after blasting must be blasted again before itis coated. As might be expected, the size and severity of the blastprofile also affects the tensile bond strength of the coating. Moreover,the temperature of the substrate can be of great consequence. It hasbeen demonstrated that a thermal sprayed coating will adhere much betterto a steel part that has been preheated to about 500° F. than to onethat is at room temperature.

In addition to problems associated with the conditions at the interface,the application of thermal sprayed aluminum is complicated by the factthat the aluminum particles adhere to one another much better than theyadhere to the properly prepared substrate. The moment the moltenaluminum droplet comes in contact with air, an oxide crust forms on itssurface. These oxide films bond very well to one another but do notadhere well to other materials. The disparity in the adhesive andcohesive strength of thermal sprayed aluminum, which is shown below inTable 1, is so large that internal stresses can actually causedisbonding or peeling of thick coatings. Consequently, the U.S. NavyManual cited above requires that no wire-sprayed aluminum coating usedaboard ship should exceed 0.010 inch (0.25 mm) in thickness. In fact, inthose applications, such as the turbocharger inlet housing and aircraftengine case described above, in which the coating is machined or abradedlater, the parts are first sprayed with a special nickel- ormolybdenum-alloy bond coat to ensure that the thermal sprayed aluminumwill not disbond or peel off the part. The dependency of bond strengthupon the condition of the metal substrate and the benefit of a bond coatare demonstrated in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Mechanical Properties of Wire Sprayed Aluminum Coatings                                      Tensile  Tensile Bond                                                                             Shear Bond                                 Surface Condition                                                                            Strength Strength   Strength                                   ______________________________________                                        90 mesh grit blast                                                                           19,500 psi                                                                             1,032 psi  2,830 psi                                  20 mesh iron grit blast,                                                                     19,500   2,142      4,500                                      warmed to 400° F.                                                      smooth surface, Mo                                                                           19,500   1,640      4,400                                      bond coat                                                                     24 pitch thread, Mo                                                                          19,500   2,400      5,500                                      bond coat                                                                     ______________________________________                                         Ref.: The METCO Flame Spray Process, METCO Inc., Westbury, NY            

Like thermal sprayed aluminum, aluminum-filled chromate/phosphatecoatings of the type described in U.S. Pat. No. 3,248,251 ('251 Allen)are durable, malleable, corrosion and oxidation resistant films. Unlikethe thermal sprayed coatings these heat cured aluminum-ceramic coatingsbond very well to steels and other metallic substrates and will greatlyextend part life in hot saline environments. Example 7 of '251 Allen,for instance, has a tensile bond strength in excess of 10,000 psi onsteel and will provide sacrificial corrosion protection at temperaturesas high as 1150° F. Consequently, aluminum chromate/phosphate coatingshave been specified on aircraft engine parts, automotive exhausts,shipboard steam handling systems, assorted fasteners, and much more.

The compositions described in '251 Allen comprise aluminum powders thataverage 5-10 microns in size. As such, the coatings of that patentcannot generally be applied at thickness greater than about 0.0015 inch(0.04 mm) per cure without significant loss of performance. Furthermore,coatings as thick as 0.01 inches (0.25 mm) or more, produced by applyingmultiple thin coats of these materials, have very poor bond strengths.

Heretofore, these limitations have excluded metal filled, e.g.aluminum-filled, chromate/phosphate coatings from most of theapplications where thermal sprayed aluminum is used, such as thosedescribed above.

The invention described herein provides a tightly adherent, corrosionand oxidation resistant metal, e.g. aluminum, chromate/phosphatecoatings that are as thick as 0.100 inch (2.54 mm) or more and that canbe applied at 0.003-0.030 inch (0.08-0.76 mm) per cure. These coatingscomprise an aqueous acidic binder, comprising metal chromate,dichromate, or molybdate ions and phosphate ions, which comprises metalpowders, like aluminum metal powders, some having an average particlesize of at least 15 μm, of which, preferably at least 5% by weight, aregreater than 44 microns in diameter (+325 mesh).

The size distribution of metal particles, e.g. aluminum particles,incorporated into the coating of this invention may be produced by usingone or more grades (or types) of powder. However, should two grades ofpowder be combined to produce the distribution, the average diameter ofthe smaller grade should be at least less than about one-half,preferably less than about one-tenth, the average particle size of thelarger powder grade, and the relative weight ratio of large to smallmetal, e.g. aluminum, powders should be preferably about 4 to 1.Coatings of this invention, containing two powders of differing sizes,in accordance with the criterion described herein are referred to asbimodal coatings and are denser, more adherent, and more corrosionresistant than those produced using a single large grade of powderalone. Such coatings have been referred to as unimodal compositions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the densert arrangement of the particle spheres.

FIG. 2 illustrates the geometrical relationship derived between theradius of the particles and the size of the interstices.

FIG. 3 shows the relationship of binder to particle when positioned inan interstice.

FIG. 4(a) shows a normal particle size distribution curve.

FIGS. 5, 6 and 7 represent tap densities for mixtures of the primary andsecondary aluminum powders.

FIG. 8 shows the bimodal powder distributions in the coatings withincreasing d/D ratio.

For a better understanding how particle size and shape can affect thedensity of a thick particulate-filled coating, it is helpful toconstruct an idealized model of the coating in which the powderparticles are represented as identical spheres. The densest arrangementof these spheres is one in which each sphere has twelve neighbors, sixsurrounding its equator, three on the plane above, and three below asillustrated in FIG. 1. Even in this configuration, the spheres occupyonly 74% of the volume of the structure. The spaces between the spheresconstitute the remaining 26% of the volume.

If the idealized coating layer is constituted of four layers of spheres,stacked one on top of the other in the close-packed manner of FIG. 1,then obviously the easiest means of producing a thicker coating is toincrease the diameter of the spheres. However, as the size of thespheres increases, so also does the size of each void, though the volumefraction of space in the structure remains constant.

The size of these interstitial voids is especially important in acomposite coating such as an aluminum-filled chromate/phosphatematerial. When the particles are small, as is the case in the coatingsof '251 Allen in which the aluminum pigments were all much less than 44microns in diameter (-325 mesh), the voids in the coating are very smalland the liquid binder fills and seals these spaces to form a densecomposite film. As the particle size increases, the voids eventuallybecome so large that the binder (which shrinks as it cures) cannot filland seal them. At this point, the apparent density of the coatingchanges dramatically. The coating, which had been a composite of smallaluminum particles with chromate/phosphate binder filling theinterstices between the aluminum, then becomes a composite of large,chromate/phosphate coated aluminum particles, interspersed with voidsand open pores.

It has been observed in the study and work done in conjunction with thedevelopment of this invention that coatings made with atomized powdersthat are greater than about 20 microns in diameter are more porous thandesired, with the adverse consequences described above. It has also beendiscovered that when the voids between powder particles in the coatingsare so large that the binder cannot fill them, they are also largeenough to accomodate another suitably sized smaller particle.

The geometrical relationships derived between the radius of theparticles and the size of the interstices (FIG. 2) require that if asmaller particle (hereafter referred to as the secondary particle) is tofit into the void without distorting the packing of the larger spheres(hereafter referred to as the primary particles), its radius should beno greater than 0.22 times the radius of the larger sphere (i.e.,r=0.22R). Such a particle, when positioned in an interstice, markedlyreduces the volume of space that the binder has to fill (FIG. 3).However, these considerations are based upon a model which assumes thateach powder particle is perfectly spherical and that each grade ofpowder consists of only one size of particle. In reality, commerciallyavailable atomized aluminum powders, especially the more commonair-atomized ones, are not very spherical and comprise a wide range ofparticle sizes.

This reality has called for additional features for the practice andmanufacturing of the compositions of the invention, as described furtherbelow.

Air-atomized grades of aluminum powder are produced by aspirating moltenmetal through a nozzle into a supersonic stream of air. The lower end ofthe nozzle dips into the bath of molten metal and its upper endterminates in a small orifice. When a jet of air is passed over theorifice of the nozzle it creates a suction effect, aspirating the liquidmetal through the nozzle, into the airstream and disintegrating themolten stream into small, discrete particles. When the stream of liquidaluminum is broken into individual droplets by the jet of air, eachdroplet is initially flattened and elongated by the force of the gasstream. Under ideal conditions, these droplets would rapidly contractinto a spherical shape in order to minimize surface area and surfaceenergy. However, when the molten aluminum contacts the air, a hard,dense oxide film immediately forms on the surface of the liquid drop.This oxide shell causes the droplet to solidify in its initial distortedshape. Consequently, air-atomized aluminum powder particles areirregular in shape and generally the smaller the particle, the greaterits variance from the perfectly spherical shape.

Special spherical grades of atomized aluminum are also commerciallyavailable, though they are much more expensive than air-atomizedmaterial. These spherical powders were developed for use in explosivesand rocket propellants where precise control of surface area isessential for control of reaction rates. Spherical powders are producedby aspirating molten aluminum into a jet of a reducing gas, such ashydrogen or an exothermic mixture of combusted methane, or an inert gas,such as helium or nitrogen. These protective atmospheres preventoxidation of the molten metal surface so that the metal droplet canrelax into its equilibrium, spheriodal shape. These grades ofgas-atomized aluminum are referred to as "spherical" powders althoughthey never actually achieve perfect sphericity because of the effect ofgravity on the molten metal droplet.

Air-atomized and non-oxidizing gas-atomized powders are typicallycharacterized according to one of the following measured parameters:particle size or average particle diameter, particle size distribution,and particle shape or configuration. Particle size, the parameter mostcommonly used to distinguish grades of powder, is generally synonymouswith particle diameter; however, particle diameter can only bedetermined accurately for spherical powders.

The average diameter of particles in any given powder grade is typicallymeasured using the Fisher Sub Sieve Sizer (ASTM B330). This devicemeasures the resistance to air flow through a packed column of powderrelative to the resistance to flow through a packed bed of sphericalparticles of known average diameter. The advantage of this technique isthat it is simple, fast and economical. However, because the processmeasures a bulk (column) property rather than monitoring individualparticles, the average particle diameter determined by the Fisher SubSieve Sizer (FSSS) is actually a statistical average rather than a trueparticle size.

Since actual atomized powder particles rarely exhibit a perfectspherical shape (for reasons mentioned above), the particle size is mostusefully established by measuring a characteristic property of anirregular particle that can be related to the same property of an idealregularly shaped particle. By choosing a sphere as the ideal shape, thesize of both air- and non-oxidizing gas-atomized powders can be reliablydescribed as "equivalent to a sphere of diameter (d)," thereby combiningthe parameters of size and shape in a single variable. An unequivocal,reproducible particle size having one dimension is thus established withthis definition.

The equivalent spherical diameters (ESD) of aluminum or other metalparticles in a particular grade of powder are measured by automatedsedimentation equipment such as the Micromeritic SediGraph 5000 Eparticle size analyzer. This device uses low energy X-rays to determinethe concentration of particles at various depths in a column of knownfluid. The laws of hydrodynamics require that the settling rate of aparticle in the fluid is related to the mass of the particle. TheSedi-Graph determines the population of particles of a particular massin the powder grade by measuring the density of particles at a givenlevel within the fluid. Since the diameter of an ideal sphericalparticle is related to its mass by means of its density and volume (i.e.diameter), each density measurement in the Sedi-Graph corresponds to apopulation count of particles with a mass that is equivalent to that ofa spherical particle having a diameter, d (designated ESD). Therefore,grades of atomized powders are completely characterized by thepopulation size distribution measured by the sedimentation technique andthe average ESD (ESD) corresponding to the median value in thedistribution.

In accordance with this invention, both air- and non-oxidizinggas-atomized aluminum powders are described using equivalent sphericaldiameter (ESD) measurements provided by sedimentation equipment.Additional information regarding analytical test methods forcharacterizing aluminum powders is provided in the Alcoa pamphletsection PAP917 (FA2D-2) entitled "Quality Control and Analytical TestMethods for Alcoa Aluminum Powders". For additional information aboutautomated sedimentation measurements, see the pamphlet entitled "A ShortCourse in Fine Particle Technology" provided by Micromeritics InstrumentCorporation. These documents are incorporated herein by reference.

Particle size distributions for three commercially available grades ofaluminum powder are shown in Table 2 below, expressed in terms ofequivalent spherical diameters. Each grade contains a substantialportion of particles which are larger than 44 microns in diameter andthe ESD of each is greater than 15 μm which is large enough to ensurethat coatings made with these grades can be applied in thicknesses of0.003 inches (0.08 mm) or more per cure. Grades A and C correspond togrades X-81 and X-75 spherical aluminum powders from Alcan AluminumCorp., Elizabeth, N.J. These powders are produced by atomizing aluminuminto a reducing mixture of N₂, H₂, CO and CH₄. Grade B is Alcan's MD101grade of air-atomized powder.

                  TABLE 2                                                         ______________________________________                                        Commercially Available Aluminum Powder                                        ESD Distribution                                                                            A         B         C                                           ______________________________________                                        10% less than  8.3 μm                                                                              13 μm  12.1 μm                                   ##STR1##      23.7      36        38.1                                       90% less than 59.8      88        89.7                                        ave. particle dia.                                                                          12-18 μm                                                                             14-22 μm                                                                             16-24 μm                                 (FSSS)                                                                        ______________________________________                                    

Each of the grades of aluminum powder shown in Table 2 exhibits either anormal or skewed particle size distribution curve (FIG. 4). Thesedistribution curves possess a single maximum frequency called a "mode"and are, therefore, termed unimodal distributions. The sums of any twoor more of these powder grades also produce a unimodal distribution.Consequently, the coatings of this invention which comprise one or moreof these coarser grades are herein referred to as unimodal coatings, aswas already referred to above.

In unimodal coatings of this invention, the ESD of the grade of thealuminum used shall preferably exceed at least 15 μm, more preferably 25μm. Furthermore, the quantity of powder exceeding 44 μm in diameter,that is that percentage greater than 325 mesh, shall preferably be atleast 5% by weight.

It is to be further noted that the density of a particulate-filledcoating is increased when the interstices between large particles arefilled with properly sized, smaller particles. To a limited extent thisdenser packing may exist in unimodal coatings due to the wide range ofparticle sizes present in commercial powder grades. In grade A above,for example, about 15% by weight of the aluminum particles are greaterthan 50 μm in diameter and about 12% of the particles are less than thecorresponding maximum interstitial size of 11 μm (0.22×50 μm).Consequently, in a coating comprising Grade A powder only, there mayexist more densely packed regions in which small particles occupyinterstices between close packed larger ones. Densification of thecoating layer can increase its corrosion resistance and bond strength;however, in unimodal coatings this effect is localized and limitedbecause a preponderance of particles in the coating do not share asimilar ideal size relationship. In fact, when less than about 5 weightpercent of the atomized powders are greater than 44 μm in diameter (i.e.greater than 325 mesh) even the localized densification effect becomesnegligible.

Another embodiment of the invention is a composition (and resultingcoating) which comprises a powder of a second, smaller particle. Thispowder effectively fills the interstitial voids.

In such compositions, virtually all the voids between the largeparticles are nearly filled with smaller particles. Four fine grades ofaluminum powder are described in Table 3 which are suitable secondarypowders for increasing the density of coating compositions of theinvention in the manner. Grades a, b and d are spherical-atomizedpowders. Grades a and b correspond to grades H-3 and H-5 helium atomizedpowders from Valimet Inc., Stockton, Calif. Grade d is Alcan's X-65reducing gas-atomized aluminum. Grade c is LSA-693 air-atomized powderfrom Reynolds Metal Co., Louisville, Ky.

                  TABLE 3                                                         ______________________________________                                        Secondary Aluminum Powders                                                    ESD Distribution                                                                          a        b        c      d                                        ______________________________________                                        10% less than                                                                             1.7 μm                                                                              2.1 μm                                                                               2.6 μm                                                                            4.1 μm                                ##STR2##    3.2      3.9       5.5    8.9                                    90% less than                                                                             6.6      8.0      10.4   16.1                                     ave. particle dia.                                                                        3-4.5 μm                                                                            4.5-7 μm                                                                            6-9 μm                                                                            4.5-9 μm                              (FSSS)                                                                        ______________________________________                                    

When in accordance with the invention, a coarser, primary powder (suchas those in Table 2), is mixed with a finer, secondary powder (such asthose in Table 3), the powder which results possesses two peakfrequencies or modes in its particle size distribution curve. Such adistribution is termed a bimodal distribution. Therefore, the coatingsof this invention which contain both small and large grades of aluminumpowder are herein referred to as bimodal coatings.

In an idealized model, the maximum allowable ratio of secondary diameterto primary diameter is 0.22. The size relationships in the twelvepossible combinations of powders from Tables 2 and 3 are listed in thefollowing tables (4, 5 and 6). In each table, the idealized maximumallowable size (i.e. 0.22 times the primary powder diameter) is listedas well as the size of each powder grade. Portions of the sizedistributions of secondary powders that are smaller than this idealizedlimit are asterisked.

                                      TABLE 4                                     __________________________________________________________________________    Comparison of Sizes of Secondary Powders Relative to Primary Powders A        ESD Distribution                                                                       A    ideal                                                                              a    b   c    d                                            __________________________________________________________________________    10% is less than                                                                        8.3 μm                                                                          1.8 μm                                                                         1.7 μm*                                                                         2.1 μm                                                                          2.6 μm                                                                          4.1 μm                                    ##STR3##                                                                               23.7                                                                                5.2                                                                               3.2*                                                                               3.9*                                                                               5.5                                                                                8.9                                        90% is less than                                                                       59.3 13.0 6.6* 8.0*                                                                              10.4*                                                                              16.1*                                        __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________    Comparison of Sizes of Secondary Powders Relative to Primary Powder B         ESD Distribution                                                                       B   .22B a    b    c    d                                            __________________________________________________________________________    10% is less than                                                                       13 μm                                                                           2.9 μm                                                                         1.7 μm*                                                                         2.1 μm*                                                                          2.6 μm*                                                                         4.1 μm                                    ##STR4##                                                                               36   7.9                                                                               3.2*                                                                               3.9*                                                                                5.5*                                                                               8.9                                        90% is less than                                                                       88  19.4 6.6* 8.0* 10.4*                                                                              16.1*                                        __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________    Comparison of Sizes of Secondary Powders Relative to Primary Powder C         ESD Distribution                                                                       C    .22C a    b    c    d                                           __________________________________________________________________________    10% is less than                                                                       12.1 μm                                                                          2.7 μm                                                                         1.7 μm*                                                                         2.1 μm*                                                                         2.6 μm*                                                                          4.1 μm                                   ##STR5##                                                                               38.1                                                                                8.4                                                                               3.2*                                                                               3.9*                                                                               5.5*                                                                                8.9                                       90% is less than                                                                       89.7 19.7 6.6* 8.0* 10.4*                                                                              16.1*                                       __________________________________________________________________________

The relative packing densities that are achievable in bimodal coatingscontaining blends of these powders can be estimated using a tap densitytest in which the powders are placed in a graduated cylinder which isthen tapped or agitated to encourage the powders to settle into theirdensest arrangements. The tap density then is the mass of powder dividedby the final volume that mass occupies in the cylinder and is expressedin terms of grams per cubic centimeter (gm/cc). This testing techniqueis amplified in ASTM B527 which is incorporated herein by reference.

The tap densities determined for mixtures of the primary and secondaryaluminum powders in the tables above are represented in FIGS. 5, 6 and7. In each curve, the tap density is plotted against the weight percentof the secondary powder in the bimodal mixture. The ratio of the ESD ofthe secondary powder to the ESD of the primary powder (designated d/D)is noted beside each curve.

All of the curves exhibit the same characteristic shape with a singlemaximum and no other points of inflection. Furthermore, the density ofthe powder blends increases as the value of d/D decreases.

The reduction in tap density with increasing d/D ratio is a reflectionof the nature of the bimodal distribution. As shown in FIG. 8, thebimodal powder distributions in the coatings of this invention are infact the sum of two distributions, each having a single mode. As long asthe population means of these two unimodal distributions are widelyseparated, the total distribution has two distinct modes and densepacking is possible. As the population means approach one another, thetwo peaks in the total particle size distribution become less distinct,fewer particles fit easily into interstices, and less densification isachieved. In the extreme, the two size distributions overlap so muchthat the total distribution begins to resemble the unimodal distributionof a single powder grade.

Consequently, in coatings of this invention containing grades ofaluminum powder of very different sizes, the ESD of the primary powder(or powders) exceeds preferably 15 μm, more preferably 25 μm, and theratio of the ESD of the smaller powder (or powders) to that of thelarger one(s) is at least less than about 0.5, and preferably less thanabout 0.12. At least about 5% by weight of the primary powder is greaterthan 44 μm in diameter, and the weight percentage of smaller powder(s)in these bimodal coatings is preferably between about 5 and 50%, mostpreferably between 15 and 25%.

The powders which are used in the compositions of this invention inaccordance with the constraints defined above may comprise either air-or spherical-atomized grades of aluminum. It is also within the scope ofthis invention that the suitably sized secondary powder be chosen from awide range of metallic pigments, such as nickel, or refractories, suchas SiC or glass, or even polymeric resins, such as acrylics orpolyesters, instead of aluminum in order to incorporate other desirableproperties into the coating film. Whenever the term "aluminum" is usedherein, it is intended to include alloys of aluminum such as MgAl, AlZnor others that are known.

It is obvious to one skilled in the art that the preferred weight ratiosof these secondary powders will be greater or less than those describedabove for aluminum, depending on the relative density of the material.

The advantage of the use of bimodal particle size distributions incompositions of this invention is clearly demonstrated when comparingthe physical properties of several coatings which use achromate/phosphate binder of the type described in '251 Allen and whichare embodiments of this invention.

One embodiment is a unimodal coating comprising Grade C powder only. Thesecond embodiment is a bimodal coating comprising powder Grades A and C(d/D=0.25) in an 80/20 weight ratio. The third coating, a preferredembodiment of this invention, contains a mixture of Grades C and a,which consist of 20% grade a by weight. The ratio of the ESD's for thesetwo powders is 0.08. The weight of aluminum per unit weight of binder isconstant for all the coatings and equal to that of Example 7 in '251Allen.

The bond strengths of these three coatings are included in Table 7 withthe bond strengths of the '251 Allen formulation which comprises smallaluminum only (ESD=6 μm). Although the unimodal coating exhibits muchgreater bond strengths than do thermal sprayed aluminum coatings, it isobvious that the addition of a secondary powder in the bimodalformulations causes a substantial improvement in the physical propertiesof the coating.

                  TABLE 7                                                         ______________________________________                                        Comparison of Thick Aluminum-Ceramic Coatings                                                     no. of  tap      bond                                     coating   thickness coats   density  strength                                 ______________________________________                                        '251 Allen, ex. 7                                                                       2-3 mils  2       1.4 gm/cc                                                                              10,000+ psi                              '251 Allen, ex. 7                                                                       18        13      1.4       1,700                                   A + c     10        2       1.795     7,500                                   C         12        3       1.746     5,800                                   C + a     13        3       2.028     9,800                                   Thermal Spray                                                                           5-10      1       --       1,000-2,000                              Aluminum                                                                      ______________________________________                                    

Since the critical component of this invention is the size and sizedistribution of the aluminum powders (or other metal powders) used inthe compositions, e.g. the primary and/or secondary powders describedherein, it will be appreciated that the particular nature of the binderportion of this invention (its ingredients and other variables) is ofsecondary importance, thus allowing a greater latitude in the selection(and proportions) of the other ingredients of the compositions for thecoatings of the invention than permitted heretofore.

The compositions of the invention are constituted or made of aqueoussolutions (or are solids as described hereinafter) of a combination ofinorganic compounds from the group consisting of phosphoric acid,chromic acid, molybdic acid, and the metal salts of these acids.Preferred solutions contain phosphate anion and chromate (or dichromate)and/or molybdate anions. A great variety of such solutions are known fortreatment of metal surfaces. For instance, Kirk and Othmer, Eds.,Encyclopedia of Chemical Technology, 2nd ed., vol. 18, IntersciencePublishers, a division of John Wiley & Sons, Inc., 1969 (pages 292-303),describes phosphate and chromate coatings. The United States patentliterature describes coating solutions or dispersions for protectivecoating of metals, which compositions are suitable for use as componentsof the compositions of the invention. Such suitable compositions aredisclosed by the Allen (U.S. Pat. No. 3,248,251); Brumbaugh (U.S. Pat.No. 3,869,293) patents referred to above; Collins (U.S. Pat. No.3,248,249); Boies (U.S. Pat. No. 3,081,146); Romig (U.S. Pat. No.2,245,609); Helwig (U.S. Pat. No. 3,967,984); Bennetch (U.S. Pat. No.3,443,977); Hirst (U.S. Pat. No. 3,562,011) patents, and others. Thesedisclosures are incorporated herein by reference. Other illustrativepatents or literature showing corrosion inhibiting and protectivecoating compositions of phosphates, mixtures of phosphates and chromatesand/or molybdates are known to one skilled in the art and furtherexamples need not be supplied.

In the binder of the chromate/phosphate composition used it is notnecessary that a metal ion be added. When the phosphate and/or chromateor molybdate ion is furnished to the solution by addition of a metalsalt, as is often done, metal ion is inherently supplied to thesolution. Hence, any of the known phosphates, chromates, dichromates ormolybdates can be used as the source of metal ion. Additionally, as isknown, metal ion can be supplied in a form such as metal oxide,hydroxide, carbonate, etc. which will dissolve in acid, i.e. phosphoricacid, chromic acid or molybdic acid, to produce the metal phosphate,chromate or molybdate, and therefore the metal ion, plus water and/orgas which is evolved. The following metal compounds will illustratethose which can be added to generate the metal ion within the solution,by an acid-base reaction, in accordance with the above: magnesium oxide;magnesium hydroxide; zinc oxide; zinc hydroxide; aluminum hydroxide;lithium oxide; lithium carbonate; and calcium hydroxide. Metal compoundswhich may be added to generate the metal ion in solution are variousoxides, hydroxides or carbonates of magnesium, zinc aluminum or others.Such procedures and sources for the metal ions are known and referencemay be made to the cited '251 Allen patent, for instance, column 7,lines 26-57, which is incorporated herein by reference.

In the coatings of the inventions, which by definition contain largealuminum particles, it is advantageous, though not necessary, to usethixotropic chromate/phosphate binders. The high viscosity of thesebinders helps to hold the larger metal pigments in suspension, makingthe coating more uniform and easier to apply. Thixotropic acidic bindersof this type may be produced by incorporating certain colloidal aluminasinto the chromate/phosphate solution, as disclosed in the copendingapplication Ser. No. 485,748, filed Apr. 18, 1983 now U.S. Pat. No.4,544,408, which is incorporated herein by reference. Alternatively, thebinder may be thickened by additions of certain amorphous silicas and anonionic surfactant. Such suitable additives and compositions aredisclosed in the copending application Ser. No. 441,754, filed Nov. 15,1982 now U.S. Pat. No. 4,548,646, which is incorporated herein byreference.

The pH of the aqueous chromate/phosphate binder used herein ispreferably, but not necessarily, in the range of about 0 to about 3.0,preferably in the range of about 1.5 to about 2.5.

It is envisioned by this invention that other binders may be used toproduce the thick metal-filled films. These include synthetic organicbinders such as silicones and phenolic resins and inorganic glasses suchas borates and other frits.

Other variations to constitute the compositions of the invention arecontemplated by the invention and can be made by one skilled in the art.

Although the principal interest is in the coating of metal parts, it isevident that non-metal parts such as ceramics, and other substrates canbe coated also. The coating can also be applied to any ferrous (iron) ornon-ferrous metal or alloy (aluminum, zinc, brass, nickel) which canwithstand the temperature required to cure the binder used in thecoating composition.

The coating compositions of this invention, therefore, comprise a liquidbinder, particularly an aqueous binder containing chromate and phosphateions, which comprise atomized aluminum powder such that the averageparticle diameter as expressed in terms of the equivalent sphericaldiameter is greater than at least 15 μm, preferably greater than 25 μm,and at least about 5% by weight, preferably about 15%, of the particlesin the size distribution are retained on a 325 mesh screen, which is tosay that they are greater than 44 μm in diameter. The particle sizedistribution may be produced by incorporating one, or more, grades ofatomized powder, each having an average ESD (ESD) greater than at least15 μm and each containing at least 5 weight percent of particles thatare +325 mesh. Such grades, either singly or in combination, yield aparticle size distribution curve with a single maximum or mode, whichconforms to the constraints of the invention as described above.

Alternatively, powders of very different sizes may be combined in asingle composition of the invention, such that the resultant particlesize distribution curve has two peak frequencies or modes. In coatingsof the invention possessing such bimodal particle size distributioncurves, it is the ESD of the larger (primary) powder(s) only that iscalled to exceed preferably at least about 15 μm, more preferably 25 μm,and the ratio of the ESD of the smaller powder to that of the larger oneis preferably at least less than about 0.5, more preferably less thanabout 0.12. Also, preferably at least about 5% by weight of the primarypowder, more preferably 15% or more, shall be retained on a 325 meshscreen. The amount of smaller (secondary) powder(s) is preferably lessthan about 50% by weight and more preferably ranges between 15 and 25weight percent.

Another embodiment of the invention provides non-aqueous, virtually drycompositions, generally, of a powder-like physical appearance andnature. These compositions comprise the metal, e.g. aluminum powdersdescribed herein and a binder, such as are described herein, which issubstantially free of water. Such a binder material may be obtained forinstance by removing the aqueous phase (as by spray-drying or otherconvenient methods from the aqueous binder) or making one without theaqueous phase and admixing all or part of the aluminum powder with theother solid materials. All or part of the dry mixture without thealuminum powder may be ball milled to a desired particle size. Thealuminum powders may be admixed with any or all of the non-aqueouscomponents and the aqueous component admixed in part or all of it toform the final or intermediate product. These compositions containideally all the necessary solids, i.e. powdery components including thealuminum powders. The aluminum powder can also be admixed to the othercomponents in part or all of it before use. In the bimodal type, eitherone of the aluminum powder (or other metal powder) may be admixed at anyappropriate time. The aqueous composition with all (or a portion of thealuminum powder if not all of it had been already incorporated) may thenbe added, then the liquid portion (all or partly) removed, as byspray-drying. The resulting composition is a dry, crumb-like material.This material may be reconstituted or brought to a desired consistencywhen desired. Thus, these steps (admixing or removing) a component canbe performed in any sequence which is desirable to achieve the objectiveintended. Such non-aqueous compositions are particularly well suited fortransporting from one location to or closer to the location where thecoating composition is intended to be applied, at which time the aqueousphase will be added together with further aluminum powder, if desired.Also, concentrates, i.e. with a high solids content, of the liquidcompositions of the invention can likewise be prepared.

The different components of the blends may be mixed to the compositionscontemplated by the invention at any time prior to use providing theblend of aluminum powder in the composition conforms to the parametersdescribed herein.

The following examples are illustrative of the invention and are notintended to be limiting. It is evident to one skilled in the art thatthe ingredients of the various compositions illustrated (e.g., theirrelative proportions and amounts), as well as other variables andparameters, can be modified while being within the scope andcontemplation of the invention.

EXAMPLE 1

A chromate/phosphate binder was prepared of the type described in '251Allen according to the following formula:

    ______________________________________                                                       Binder A                                                       ______________________________________                                        3200 gm          Deionized water                                              1525 gm          Phosphoric acid, 85%                                          350 gm          Chromic acid                                                  300 gm          Magnesium oxide                                              ______________________________________                                    

A preferred embodiment of the bimodal aluminum-filled ceramic coatingsdisclosed herein was then prepared according to the followingproportions:

    ______________________________________                                        385    gm       Binder A                                                      430    gm                                                                                      ##STR6##                                                     110    gm                                                                                      ##STR7##                                                     10     gm       Fumed silica (Cab-O-Sil M-5)                                  0.6    gm       Nonionic surfactant (Triton X100)                             ______________________________________                                    

The weight percentage of secondary powders in this formulation was 20%which was equal to that percentage which produced the maximum density intap density tests. The ratio of average particle diameters was 0.08.

Ten two-inch by four inch panels of low carbon sheet steel and six,one-inch diameter, half-inch thick disks of 4130 steel were degreased inhot solvent and then grit blasted with 90-120 mesh alumina. Four coatsof the bimodal coating were then sprayed onto these clean, blasted testspecimens, producing 0.010-0.015 inch thick films. Each coat was curedone-half hour at 650° F. (343° C.). After the final coat was cured,three of the coated disks were heated at 1000° F. (580° C.) for 90minutes by which time the aluminum-ceramic film had become electricallyconductive. The electrical resistance of this postcured coating was only2 ohms across a one-inch probe gap.

Six of the coated steel panels were lightly abraded with alumina gritafter curing of the final coat. This burnishing operation also made thecoatings on these panels electrically conductive. Three of these gritburnished panels were then topcoated with an aqueous chromate/phosphatesealer essentially consisting of binder A above and fine ceramicpigments (4-6 μm). The topcoat was also cured for one-half hour at 650°F. (343° C.).

The corrosion resistance of the bimodal coating was excellent whentested in 5% salt spray per ASTM B117, no matter what the condition ofthe coating film. The postcured coatings and the grit burnished coatingsexhibited only slight quantities of white, sacrificial, corrosionproducts after 500 hours of exposure to the salt fog. There was no redrust on the panels, not even in scribe marks which had been cut throughthe coating to the surface of the panel.

After 500 hours in salt fog, there was no evidence of either red orwhite corrosion products on panels on which the bimodal coating had beengrit burnished and sealed with the chromate/phosphate topcoat.Surprisingly, similar inactivity was observed on panels which had notbeen post treated in any way--even though the coatings had been scribedbefore being placed in the salt fog. The sacrificiality which preventedrust from forming in the scribe on these electrically non-conductivepanels had heretofore been observed only in aluminum-filledchromate/phosphate coatings which had been thermally or mechanicallypost treated to achieve electrical conductivity.

Two of the grit burnished panels, one topcoated, one not, were alsosubjected to heat/salt cycling tests. In each cycle, the panels wereheated in air for six hours at 750° F. (399° C.), cooled at roomtemperature for two hours, and then placed in a 5% salt fog (ASTM B117)for sixteen hours. Even after ten of these severe, combined oxidationand corrosion cycles, neither panel exhibited signs of rust or whitesacrifical product though both had darkened slightly.

The coated one-inch diameter disks were used to determine the tensilebond strength of this bimodal coating in accordance with the methoddescribed in ASTM C633. Both sides of the disk were coated with an epoxyadhesive and it was sandwiched between the ends of two one-inch diameterrods, the other ends of which were threaded. The rods and disk werepositioned in a fixture so that all were aligned along the same axis andthe assembly was heated to cure the epoxy. When cooled, the ends of therods were threaded into grips in a mechanical testing machine and thespecimen was pulled apart at a rate of 0.10 inches per minute.

The average measured tensile bond strength of the three coated and cureddisks was 9800 psi. Postcuring the coating at 1000° F. for 90 minutesreduced the bond strength to 9000 psi.

In addition to providing durable, oxidation and corrosion resistance tometal parts, this bimodal formulation could be used to repair or restorethe surface finish of hardware. This capability was demonstrated on abadly pitted compressor blade from a gas ground turbine unit. Some pitson the airfoil surface of the blade were small (0.010 inches indiameter) but deep (0.080 inches). Others were shallow (0.015 inches)but wide (0.50+ inches in diameter).

The blade was blasted with clean 90-120 mesh grit at about 40 psi toremove all the corrosion deposits from the pitted areas. Then a coat ofthe bimodal slurry was spatulated onto the airfoil. This material wasforced into the deep pits and smoothed over the shallow ones so thatimperfections were completely filled. The coating was allowed to dryovernight at 175° F. (79° C.) before it was cured at 650° F. (343° C.).

After the cured coating had cooled, the airfoil surface was ground witha belt grinder to restore the blade contour. A second coat of thebimodal coating was sprayed on, cured and ground to produce a smooth,uniform surface. Finally, a coat of a material containing only finealuminum (like Example 7 in '251 Allen) was applied, cured and polished.The result was complete restoration of the contour and finish of theairfoil surfaces of the pitted blade.

EXAMPLE 2

The coating in Example 1 was repeated, omitting the fumed silica andnonionic surfactant additions. Tensile bond strengths determined as inExample 1 were calculated to be approximately 9000 psi. Also, mild steelpanels when coated with 10-15 mils of the coating showed no signs ofcorrosion after 500 hours of 5% salt spray exposure per ASTM B117.

EXAMPLE 3

The following thixotropic coating was prepared by mixing the ingredientsin a high speed blender for 10 minutes:

    ______________________________________                                        385    gm     Binder A                                                        430    gm                                                                                    ##STR8##                                                       110    gm                                                                                    ##STR9##                                                       27     gm     Strontium chromate                                              10     gm     Cab-O-Sil M-5                                                   0.6    gm     Triton X-100                                                    ______________________________________                                    

Coated mild steel specimens when placed in a beaker of tap water or 5%salt solution for 500 hours showed no signs of corrosion.

EXAMPLE 4

A thixotropic coating was prepared using only one type of aluminumpowder pigment at a level that maintained the same pigment/binder solidsratio as the previous examples. The following ingredients were blendedfor 10 minutes:

    ______________________________________                                        385    gm     Binder A                                                        540    gm                                                                                    ##STR10##                                                      10     gm     Cab-O-Sil M-5                                                   0.6    gm     Triton X-100                                                    ______________________________________                                    

Tensile bond strengths of the coating were determined per ASTM C633. Thebond strength of the coating in an as cured condition averaged about7200 psi. When the specimens were postcured at 1000° F. for four hoursthe tensile bond strength was determined to be about 6000 psi.

EXAMPLE 5

A thixotropic coating similar to Example 1 was made, substitutingdispersible alumina for the fumed silica and nonionic surfactantadditions. The formula is given as follows:

    ______________________________________                                        385    gm     Binder A                                                        480    gm                                                                                    ##STR11##                                                      60     gm                                                                                    ##STR12##                                                      50     ml     10% Dispural alumina dispersion in                                            H.sub.3 PO.sub.3 (made by mixing 6 ml 50%                                     H.sub.3 PO.sub.3, 444 ml water and 45 gm                                      Dispural boehmite).                                             ______________________________________                                    

Mild steel test panels coated with 10-15 mils of the coating showedexcellent results when tested in salt spray, tap water immersion andsalt water immersion for 500 hours.

EXAMPLE 6

Example 5 was repeated adding in addition 5 grams of fumed alumina.

Coated mild steel test panels were scribed with a one inch line throughto the substrate. The specimens showed excellent corrosion resistanceafter 500 hours in 5% salt spray.

EXAMPLE 7

A coating similar to Example 1 was manufactured. Two air-atomizedaluminum powders were used in place of the spherical powders of thefirst example. The following ingredients were mixed in a high shearblender:

    ______________________________________                                        385    gm     Binder A                                                        430    gm                                                                                    ##STR13##                                                      110    gm                                                                                    ##STR14##                                                      10     gm     Cab-O-Sil M5                                                    0.6    gm     Triton X-100                                                    ______________________________________                                    

Tensile bond strengths for the coating were determined to beapproximately 7500 psi.

The superficial hardness of this coating can be increased by replacingall or part of the Reynolds LSA-693 aluminum with similarly sizedparticles of a hard refractory material such as silicon carbide (SiC) ora hard ceramic such as aluminum (Al₂ O₃). Alternatively, aself-lubricating coating can be produced by substituting 5 μm particlesof a lubricious solid such as polytetrafluoroethylene (PTFE) ormolybdenum disulfide (MOS₂) for the Reynolds aluminum.

EXAMPLE 8

A coating was prepared using a binder composition of the type disclosedby Wydra (U.S. Pat. No. 3,857,717) and given by the following formula:

    ______________________________________                                                        Binder B                                                      ______________________________________                                        600 gm            Deionized water                                             170 gm            Phosphoric acid                                             110 gm            Phosphorous acid                                            140 gm            Chromic acid                                                ______________________________________                                    

The coating contained two aluminum powders in an optimum ratio withrespect to tap density. It was mixed in a blender according to thefollowing formula:

    ______________________________________                                        385 gm   Binder B                                                             400 gm   Reducing gas-atomized spherical                                               aluminum powder, Alcan X-75, 38.1 um ESD                              80 gm   Helium-atomized spherical aluminum                                            powder, Valimet H-5, 3.9 um ESD                                      ______________________________________                                    

Mild steel test panels, when coated with 10 mils of the coating appliedin four coats, each cured at 650° F., showed little or no change whenimmersed in tap water for extended periods of time.

EXAMPLE 9

Another binder with no added cation was prepared by mixing the followingingredients:

    ______________________________________                                                        Binder C                                                      ______________________________________                                        1200 gm           Deionized water                                              215 gm           Phosphoric acid, 85%                                         90 gm            Chromic acid                                                ______________________________________                                    

A thixotropic coating composition was prepared according to thefollowing:

    ______________________________________                                        385    gm     Binder C                                                        430    gm                                                                                    ##STR15##                                                      110    gm                                                                                    ##STR16##                                                      ______________________________________                                        Mix in blender for 10 minutes, than add:                                      10     gm     Fumed silica, Cab-O-Sil M5                                      0.6    gm     Triton X-100                                                    ______________________________________                                    

The coating exhibited excellent resistance to long term 5% salt waterimmersion when tested on AISI 1010 steel panels at a thickness of 15mils.

EXAMPLE 10

A binder composition similar to that used Example 1 was prepared,substituting zinc oxide for the magnesium oxide. Suitable zinc oxide wasadded to achieve a pH of 1.5. The binder (binder D) was used in thefollowing coating composition:

    ______________________________________                                        400 gm   Binder D                                                             225 gm                                                                                  ##STR17##                                                           110 gm                                                                                  ##STR18##                                                           When tested for 500 hours in 5% salt spray per ASTM B117, the coating had     no signs of corrosion as applied on 1010 steel test panels at a thickness     of 12 mils in four separately cured coats. The coating was cured at       

The following is an embodiment of the bimodal coatings discussed hereincontaining a secondary particle that is other than aluminum:

    ______________________________________                                        385    gm     Binder A from Example 1                                         280    gm                                                                                    ##STR19##                                                      250    gm                                                                                    ##STR20##                                                      10     gm     Fumed silica (Cab-O-Sil M-5)                                    0.6    gm     Nonionic surfactant (Triton X-100)                              ______________________________________                                    

This formulation was blended at high speed for 10 minutes. Thethixotropic binder was used to retard settling of the much heaviernickel pigment. The added density (8.9 gm/cc) of the secondary nickelpigment also required that a greater weight had to be added to achievethe same effect as a 20% addition of similarly sized aluminum powder(density=2.7 gm/cc).

Other secondary powders, such as atomized iron, chromium or theiralloys, can be substituted for or added to the nickel powder specifiedin this composition.

We claim:
 1. A coated metal or coated ceramic part which exhibitsimproved salt corrosion and oxidation resistance, which part is coatedwith a cured coating composition or a cured binder which comprisesphosphate ions and ions of the group of chromate or molybdate ions, andan atomized aluminum powder having an average particle size (asexpressed in terms of the median equivalent spherical diameter (ESD),that is greater than at least 15 μm, and having a size distribution suchthat at least about 5% of the particles by weight are retained on a 325mesh screen.
 2. The metal part of claim 1 which is a thick multilayercoating, each layer being from about 0.003 to about 0.030 inch thick. 3.The coated part of claim 1 wherein the particle size distribution of thealuminum has a single maximum, due to the presence of one or more gradesof atomized powder, each having an ESD greater than 15 μm.
 4. The coatedpart of claim 1 wherein the particle size distribution curve of thealuminum has a particle size distribution curve, which curve has twopeak frequencies which are attributable to the presence of a mixture oflarger particle size powder which has an average particle size greaterthan 15 μm and in which the size distribution is such that at leastabout 5% by weight of the particles are greater than 44 microns, and ofat most 50 weight percent a mixture of smaller particle size powder, thetwo powders being in such ratio that the ESD of the smaller to thelarger powder is less than 0.5, the average particle size ofthe powdersbeing expressed in terms of the median eqivalent spherical diameter,ESD.
 5. The coated part of claim 1 wherein the weight percentage of thepowder of smaller particle size is between 15 and 25 μm.
 6. The coatedpart of claim 1 wherein the ratio of the ESD of the powder of smallerparticle size to that of the larger particle size powder is less than0.12.
 7. The coated part of claim 1 wherein the ratio of the ESD's ofpowders of smaller and larger particle size is less than 0.12 and theweight percentage of the powder of smaller particle size is between 15and
 25. 8. The coated part of claim 1 wherein the particle sizedistribution curve has two distinct peak frequencies attributable to thepresence of a powder with an ESD particle size greater than 15 μm and atmost 50 weight percent of a small particle size powder, selected suchthat the ratio of the ESD of the later powder to that of the formerpowder is less than 0.5.
 9. The metal part of claim 1 which is dieselengine turbocharger inlet housing.
 10. The metal part of claim 1 whichis an aircraft engine case.
 11. The metal part of claim 1 which is acompressor case of an aircraft turbine engine.
 12. The metal part ofclaim 1 which is a compressor blade or an aircraft turbine engine. 13.The metal part of claim 1 which has a ferrous surface.
 14. The metalpart of claim 1 the coating of which is at least about 0.100 inch thick.15. The metal part of claim 1 which is abradable.
 16. The coated part ofclaim 1 wherein the weight percentage of the powder of smaller particlesize is between 15 and 25, the ratio of ESD's of the powder of smallerand larger particles size is less than 0.12 and the radius of the smallparticles is not greater than 0.22 times the radius of the largerparticles.
 17. The coated part of claim 16 wherein the average diameterof the smaller powder is at least about one-half of the average particlesize of the particles of the larger particles of the powder.
 18. Thecoated part of claim 17 wherein the relative weight ratio of the largerto the smaller particles of the powders is about 4 to
 1. 19. The coatedpart of claim 18 wherein the average particle size of the smallerparticles of the powder is at least about one-tenth of the averageparticle size of the larger particles of the powder.
 20. The metal partof claim 1 wherein the aluminum powders are spherical powders.
 21. Themetal part of claim 20 wherein the spherical aluminum powders areair-atomized powders.